1. Field of the Invention
The present invention relates to a tunable antenna structure. In particular, the present invention relates to a tunable antenna structure that uses a variable capacitor to adjust the operation frequency.
2. Description of Related Art
The GSM standard is the most mature and widely adapted wireless communication standard today. According to the statistics from the GSM association, GSM technology occupies ⅓ of the worldwide wireless communication market currently and still increasing. In addition to voice communication, GSM also provides data communication solutions for middle and low speed data transfer, for example, GPRS/EDGE, Evolved EDGE, etc, to meet the band width requirement for such data transmission. The GSM standard emphasizes on the development of software technology rather than relying on the renewal of network hardware devices.
In order to meet the requirement of an increased data transmission rate, such as video-audio data sharing and instant video communication, the 3 G wireless communication technology has been developed. Currently, 3 G technology includes the European UMTS technology standard (such as WCDMA and TD-CDMA) and the American CDMA2000 technology standard. With the maturity of the HSDPA, HSPA, HSPA+Rel.8 protocols, the UMTS standard originally developed form GPRS and EDGE has further evolved into the 3.9 G long term evolution (LTE) standard. As 4 G becomes widely adapted, the quantity of the UMTS/HSPA data package has greatly surpassed the quantity of the traditional voice package, and is still growing at a rate four times of the increasing rate of the voice package. Moreover, the LTE enables high bandwidth download rate at 100 Mbps and upload rate at 50 Mbps, and has improved spectral efficiency. Thus, LTE could be a promising solution for meeting the high data transmission rate requirements and improving the transmission delay problems.
However, in order to miniaturize the dimension of LTE antenna, the substrate with small dimension is adopted. The problem of the band width for low frequency of antenna is inadequate. The operation frequency cannot be fully covered.